Solid state structure of NH-pyrazoles not easily amenable to crystal structure determinations: The case of 3(5)-phenyl-5(3)-methylpyrazole and 3,5-diphenyl-4-methylpyrazole

The structures of 3(5)-methyl-5(3)-phenylpyrazole (polymorph B ) and 3,5-diphenyl-4-methylpyrazole in the solid state cannot be determined by X-ray crystallography due to the lack of suitable monocrystals. The combined use of CPMAS nmr, DSC and powder diffraction provides information about the behaviour of these pyrazoles in the solid state, particularly, about N-H ?N intermolecular proton transfer. 3(5)-Methyl-5(3)-phenylpyrazole is an example of the influence of polymorphism on the proton exchange since polymorph A (a tetramer formed by a mixture of both tautomers) presents the phenomenon but polymorph B (formed exclusively by 3-phenyl-5-methyl tautomer) is devoid of it.     

Beiträge zur Chemie der Silicium-Stickstoff-Verbindungen, 166. Mitt.: Organylaminosubstitutionen an Hexachlordisiloxan

Summary Reaction of hexachlorodisiloxane with primary and secondary amines leads — in dependence of stoichiometry — to numerous partially and totally organylamino substituted disiloxanes. Partially aminosubstituted chlorodisiloxanes are very sensitive to moisture and can be converted into disiloxanes with different organylamino groups. Exhaustive alkanolysis substitutes amino as well as chloro groups giving hexaalkoxydisiloxanes, but partial alkanolysis may substitute amino in preference to chloro groups. Mass spectra can be interpreted by abstraction ofRRN·,RRN⁺,RRNH and (RRN minus H) units. Most of the compounds are colourless liquids but hexakis(piperidino)-disiloxane melts at 270 °C.

Gewidmet Herrn Prof. Dr. Edwin Hengge zum 60. Geburtstag     

Correlation of ordering formation and surface structure in (GaIn)P using modulated MOVPE

Modulated metalorganic vapour phase epitaxial growth (MOVPE) is used to clarify the role of the surface conditions on the ordering behaviour in ternary (GaIn)P layers. The alternating deposition of GaP and InP layers with individual thicknesses of up to one monolayer is successfully used for the growth of (GaIn)P bulk layers lattice matched to (100) GaAs substrates with various off-orientations. The layer quality and the degree of ordering are investigated using high-resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL), respectively. The application of modulated growth conditions for the deposition of (GaIn)P bulk layers has a strong influence on the degree of ordering achieved in the intermediate growth temperature regime where the highest degree of ordering occurs under continuous MOVPE. Beside a new boundary structure observed in layers grown under modulated flux conditions, the successful growth of highly ordered (GaIn)P layers grown using the modulated MOVPE technique support the model that up to 2 monolayers of the (GaIn)P growth surface are involved in the ordering formation process.     

Coupling of functional hydrogen bonds in pyridoxal-5'-phosphate-enzyme model systems observed by solid-state NMR spectroscopy

We present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base models of the cofactor pyridoxal-5'-phosphate (PLP) with carboxylic acids that mimic the cofactor in a variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H-15N couplings of static samples of the deuterated compounds. An interesting coupling of the two "functional" OHN hydrogen bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen atom carries an aromatic substituent.     

Nacnac(Bn)MgOtBu: a diketiminate-based catalyst for the polymerisation of rac-lactide with slight isotactic preference

The reaction of N,N'-dibenzyl-2-amino-4-imino-pent-2-ene, nacnac(Bn)H, with 1 or 2 equiv. of MgnBu? afforded the homoleptic complex nacnac(Bn)?Mg. The reaction of nacnac(Bn)H with Mg(N(SiMe?)?)? yielded nacnac(Bn)MgN(SiMe?)?, which reacted with tert-butanol to form nacnac(Bn)MgOtBu. The latter complex crystallizes as an alkoxide bridge dimer and is active in the ring-opening polymerisation of rac-lactide. Polymerisations at room temperature afforded atactic polylactide, while polymerisations at -17 and -26 °C afforded polylactide with a small isotactic bias (P(m) = 0.52, and 0.55, respectively).     

Probing the antiknock effect of anisole through an ignition,speciation and modeling study of its blends with isooctane

In order to unravel the reaction pathways relevant to anisole co-oxidation within a fuel blend, a detailed study of isooctane/anisole blends was performed with the ULille RCM. Ignition delays as well as mole fraction profiles were measured during a two-stage ignition delay using sampling and GC techniques. These results are used to validate a kinetic model developed from ab initio calculations for the most relevant rate constants which included H-atom abstraction reactions from anisole, and reactions on the potential energy surfaces of methoxyphenyl + O₂ and anisyl + O₂. Pressure dependent rate constants were computed for the methoxyphenyl + O₂ and anisyl + O₂ reactive systems using master equation code analysis. The new kinetic model shows good agreement with the experimental data. Dual brute-force sensitivity analysis was performed, on both first- and second-stages of ignition, allowing the identification of the most important reactions in the prediction of both ignition delays. It was observed that while pure anisole does not show NTC behavior, a 60/40 isooctane/anisole blend displays such behavior, as well as two-stage ignition. This suggests anisole addition may not be as beneficial to knock resistance as expected from its high octane number. The kinetic modeling results demonstrate the importance of H-abstraction reactions both from the methoxy group and from the aryl ring in ortho-position and the addition of the resultant radicals to O₂, mostly leading to the formation of polar or non-aromatic products.     

Understanding MOF Flexibility: An Analysis Focused on Pillared Layer MOFs as a Model System

Flexible porous frameworks are at the forefront of materials research. A unique feature is their ability to open and close their pores in an adaptive manner induced by chemical and physical stimuli. Such enzyme-like selective recognition offers a wide range of functions ranging from gas storage and separation to sensing, actuation, mechanical energy storage and catalysis. However, the factors affecting switchability are poorly understood. In particular, the role of building blocks, as well as secondary factors (crystal size, defects, cooperativity) and the role of host–guest interactions, profit from systematic investigations of an idealized model by advanced analytical techniques and simulations. The review describes an integrated approach targeting the deliberate design of pillared layer metal–organic frameworks as idealized model materials for the analysis of critical factors affecting framework dynamics and summarizes the resulting progress in their understanding and application.